The present invention is in the field of semiconductor device fabrication, and in particular, relates to the radiation hardening of microcircuits.
In standard microelectronics technology one of the finally processes undergone is an anneal of the finished circuit in forming gas (a mixture of hydrogen and nitrogen or argon) at temperatures in the range 380° C. to 430° C. for periods of up to 30 minutes. Rapid thermal annealing has also been used. The primary objective of these anneals is to passivate the interface (dielectric/semiconductor) of the metal-oxide-semiconductor field effect transistors (MOSFETs) in order to enhance the carrier mobility in the inversion channel of the transistor and to eliminate threshold voltage shifts due to the presence of interface states. The subsequent depassivation of the interface (involving removal or release of the bonded hydrogen atoms attached during the passivation anneal) by hot carrier injection from the inversion channel during normal operation is the process by which degradation and aging of the device occurs. Failure of the device is usually observed when the hot electron induced degradation results in device channel mobility or threshold voltage shift outside a range of values considered acceptable.
It is known that if the gas used during the passivation anneal is one in which the hydrogen component is replaced by deuterium, then the resistance of the transistor dielectric/semiconductor interface to hot electron degradation is substantially increased. This resistance to hot electron degradation is specifically related to replacement of silicon-hydrogen bonds at the interface by silicon-deuterium bonds.
U.S. Pat. No. 6,143,632 addresses the problem of hot carrier degradation at the interface between the silicon substrate and the SiO2 gate oxide layer by introducing deuterium before the uppermost conductive layer is formed, i.e., the gate oxide is grown in a D2O vapor atmosphere which then diffuses through the gate oxide to the Si/SiO2 interface. The final annealing step is performed in a deuterium atmosphere at about 400 to 550° C. for 30 minutes.
Other research has addressed the issue of electrical stress induced leakage currents (SILC) through the gate dielectric itself of the MOSFET. In this case, electrical charge is injected into the gate dielectric by application of a large electric field between the gate electrode and the substrate/source/drain contacts of the device. The mechanism invoked is the so-called Fowler-Nordheim tunneling. This becomes significant only when the electric field exceeds values of about 4 MV cm−1. Device operating voltages are usually such that this regime of operation is avoided. In this case, it has been demonstrated that annealing of the finished devices in deuterium containing gas can result in an improved resistance to SILC. If the dielectric itself (SiO2) is grown in a wet atmosphere (D2O+O2) then additional improvements in resistance to SILC can be obtained.
There is an important difference between radiation hardening a circuit and hardening a particular device within a circuit to improve resistance to hot carrier degradation or SILC. A circuit is comprised of three important areas where there are dielectrics that can be a source of radiation sensitivity. These areas are: (a) the gate oxide of the device (elemental transistor); (b) the field or isolation oxide (isolating one device from its neighbor); and (c) the isolation layer between interconnect lines (usually metallic) which link device to device or device to the outside world. For most purposes it is the field or isolation oxide (b) which is the most important in radiation hardness. The gate oxide (a) is most important in device reliability and lifetime. The isolation layer between interconnect lines (c) may be important for overall circuit failure, but its importance for radiation hardness is unknown.
Current annealing techniques are directed toward reducing the hot carrier degradation and the SILC problems of specific semiconductor devices. They are not directed toward increasing the resistance of the overall circuit to damage caused by external radiation. Accordingly, there is a need for an annealing technique that can accomplish this and in particular that can improve the radiation hardness of the field or isolation oxides used throughout semiconductor circuits.